In an Ethernet local area network environment, collisions are utilized to provide management of access to the network between stations. Thus, a carrier sense multiple access (CSMA). System is employed wherein stations observe the network for the presence of carrier before sending and if such carrier is present, the stations defer for a period of time after the carrier is no longer present on the network before beginning a transmission. However, because of propagation delay along the network, a given station can begin transmitting within a time window without being able to detect the start of transmissions of other stations on the network. Therefore, a collision can occur wherein the transmissions from the two stations interfere with each other. The Ethernet local area network uses collisions and "backoff" wherein stations that have participated in a collision will defer a random amount of time before attempting retransmission after a collision has occurred.
In order to effectively install and maintain a network, it is desirable to be able to observe the various type and frequencies of collisions to assist in analyzing for potential problems. If the number of collisions becomes excessive, network throughput can be degraded. FIG. 1 is a block diagram of a typical Ethernet employing two segments 10,12, wherein each segment is a maximum of 185 meters in length with a repeater 14 providing interconnection between two segments to effect data transmission therebetween. Each segment has a number of nodes or stations 16 thereon which send and receive traffic on the network. Each end of an Ethernet segment is terminated by a 50 ohm terminator 18 which provides a termination impedance equal to the characteristic impedance of the network cable. In accordance with prior art network analysis devices, collision and frame errors are individually counted wherein a collision may produce a count as a collision as well as a count as a frame error (since the collision likely resulted in errors in the particular frame or frames that were collided with). This multiple counting of the same error event can obscure pertinent information about the probable cause of collisions or errors, since a simple collision is likely to result in frame error counts which can confuse the network technician attempting to analyze and locate the problem.
The above example assumes a 10Base2 network. On a 10BaseT network, every link to a local repeater is itself a separate segment and terminates at the repeater and the local node, so most collisions would be classified as remote collisions unless the particular node is the colliding station.
Since collisions can rob a network of available bandwidth without any apparent symptoms to the casual user, it is desirable to provide a quantitative analysis of collisions as a percentage of the bandwidth available to enable the network technician to quickly analyze the situation.
The Ethernet and IEEE 802.3 specifications require that all frames on the network begin with a specific pattern known as a preamble and Frame Start Delimiter. Frames not meeting this requirement, referred to herewithin as "ghost" frames, are assumed to be fragments of collisions which are normal and insignificant and are therefore ignored by normally operating stations in accordance with the Ethernet specification. Network interface circuitry used by prior art local area network test equipment ignores such frames, since the Ethernet specification sets forth that such frames should be ignored by a receiving station. However, as the number of ghost frames becomes significant in terms of bandwidth consumption, large amounts of network bandwidth can be wasted without detection, robbing a network of bandwidth without any visible signs thereof. Ghost frame problems have been traditionally difficult to isolate and repair. Other problems can arise in a network wherein noise on the network (especially 60 Hertz noise) can also consume bandwidth just as collisions would as a result of network devices reacting to the noise. However, the potential impact of noise on users is far greater than that of collisions because the frequency of occurrence of noise could be far greater than the number of collision events would be.
Many forms of noise can exist on a network segment without noticeably hampering network throughput or functionality. Some types of noise will fool nodes on a network segment into thinking they are receiving a frame; all nodes will react differently, since there are no standards defining how or when a node should react to a noisy LAN segment. Grounding problems and noise problems are the most common sources of ghost frames. For example, ground loops and other wiring problems will cause some repeaters to believe that a frame is being received. Since the repeater is only reacting to an AC voltage riding on the cable, there will not be a valid frame to pass on to the other ports. The repeater however will transmit something on the other ports. This may either be a jam pattern or a very long preamble. Ghost events on a LAN do consume bandwidth and can slow a network down. Prior art network analyzers do not recognize such ghost events.